Wound healing is a complex process of cellular and biochemical overlapping events that occurs in four phases—hemostasis, inflammation, proliferation, and remodeling—wherein any delay or failure in one of these phases may lead to impaired healing or failure of a wound to close. The hemostasis phase may be thought of as the initializing step for wound healing. During this phase, damaged blood vessels are essentially sealed by constriction (eventually followed by local vasodilation) and a stable clot is formed to maintain hemostasis. Platelets are activated by collagen from subendothelial layers to change the platelet structure and function, which induces thrombin, fibrin, and hemostatic clot formation.
During the inflammatory phase, the body's immune system response may be characterized by increased vascular permeability, local increase in cytokines and growth factors, attraction and activation of migrating cells. The major goals of the inflammatory phase are to provide for hemostasis and promote removal of cellular, extracellular and pathogen debris. Inflammatory neutrophiles, monocytes, fibroblasts and endothelial cells are recruited to the injury site during this phase. By the third day after an injury, neutrophiles begin to be replaced with activated macrophages, which provide many functions in wound repair, including production of nitric oxide (NO). Enzymes released by macrophages (collagenase and elastase) contribute to debridement of injured tissue, yet a large number of released growth factors stimulate formation of new vessels, chemoattract fibroblasts and induce their proliferation. In acute wounds, the inflammatory stage usually lasts three days. By that time, the absence of any pathogens and devitalized tissue allows the wound to progress to the next phase. In humans with diabetes, reduced activation of inflammatory responses and decreased chemotaxis leads to decline in collagen formation and more subsequent infections. On the other hand a prolonged inflammatory phase may induce hyperproliferative scaring due to increased stimulatory effect of excess released cytokines and growth factors.
During the proliferation phase, epithelialization, neoangiogenesis and collagen deposition by fibroblasts are major components that contribute to formation of granulation tissue. Clinically, pebbled red tissue in the wound base can be seen and can generally involve the replacement of dermal tissues (and sometimes subdermal tissues in deeper wounds) as well as contraction of the wound. Epithelial cells infiltrate to the wound site, where they proliferate and differentiate to form neo-epidermis, thereby providing a protective barrier against infections and fluid loss. The major components of granulation tissue are newly synthesized connective tissue and capillary loops. Fibroblasts, keratinocytes, and endothelial cells collaborate to form new vessels and connective tissue.
The differentiation phase of wound healing begins after termination of cell proliferation and processes associated with the formation of new vessels. The main aspect of this stage of wound repair is the deposition of collagen in the injured site. During this phase, simultaneous synthesis and lysis of extracellular matrix components take place. Macrophages, epidermal cells, and endothelial cells, as well as fibroblasts release several proteomic enzymes such as MMP-1 (collagenase), MMP-2, MMP-9 (gelatinase), MMP-3 (stromelysin) that are involved in old matrix breakdown. In addition, fibroblasts secrete inhibitors TIMP-1 and TIMP-2 that protect matrix synthesis against proteolytic activity of mellatoproteinases. Newly formed collagen is more organized and thicker, providing more tensile support for the connective tissue. An imbalanced ratio between these two groups of components can complicate wound repair. For instance, elevated MMP levels can impair synthesis and deposition of new matrix proteins.
During the complex process of wound healing, gas transmitters are generated within the human body and these can play significant roles in the healing process. Two examples of such gas transmitters are NO and carbon monoxide (CO). NO is generated within the body by nitric oxide synthase (NOS) in the oxidation of L-arginine:L-arginine+O2→NO+L-citruline  (1)
NO can act as a messenger molecule in a wide range of biological processes, such as vascular smooth muscle relaxation, neurotransmission, and inhibition of platelet aggregation. Additionally, NO can play a key role in wound healing by acting as a regulator to control epithelialization, angiogenesis, and collagen deposition. Low levels of NO produced by endothelial NOS (eNOS) and neuronal NOS (nNOS) can be beneficial during the inflammatory phase of wound healing. In contrast, sustained high levels of NO produced by inducible NOS (iNOS) in macrophages may impair wound healing due to oxidation of NO to peroxynitrite (ONOO−), which is a cytotoxic molecule associated with apoptosis and necrosis.
NO and ONOO− are among the molecules that can induce heme oxygenase to generate CO within the body:Heme+O2→biliverdin+Fe2++CO  (2)
The generated CO may also play a role in wound healing by activating guanyl cyclase, which leads to smooth muscle relaxation and other vascular effects in tissues. CO can act as a messenger molecule in a wide range of biological systems and is involved in platelet aggregation, neurotransmission processes, protection against oxidative injury and cell death, and it has anti-inflammation activity. CO can influence gene expression in hypoxia, which is a physiological regulator of erythropoiesis, angiogenesis, glycolysis and tissue remodeling. CO can also cause a release of NO from heme that may result in activation of guanylate cyclase.
Oxygen (O2) also has a role as a catalyst and energy source for many cellular functions including maintenance, metabolism and repair. Cells that utilize O2 in the aerobic metabolism of glucose can generate ATP, which can fuel the active cellular process such as the ones during wound healing. Also, O2 can reduce the uncoupling of eNOS and reduce production of superoxide (O2−) and ONOO−, which are the main components of oxidative and nitroxidative stress.
Despite advances in understanding of wound healing, there remains a need for therapeutic devices and/or methods that affect the mechanisms of wound healing, such as vasodilation, inflammation, expression of matrix metalloproteinases, apoptosis, bacterial growth, and collagen deposition. Moreover, there remains a need for improved devices and/or methods of topically treating wounds.